The invention relates to mining and in particular to a method of pitless extraction of mineral resources occurring as a series of seams, say, coal and oilshale seams.
The method in compliance with the invention may be used to best advantage in underground gasification of coal seams occurring at a pitch angle of up to 40.degree..
The proposed method may be also used in underground gasification of oil shales occurring as a series of seams.
At the present time underground gasification techniques are used to gasify single coal seams or a series of individual coal seams. Most of coal formations are composed of a series of seams. To gasify all seams in the series, the techniques involved should ensure a decrease in rock deformation associated with the gasification of the seams comprised in the series. Otherwise some coal seams may not be gasified or a gaseous medium and gas may penetrate to the surface of the earth, a factor violating a normal production process and contaminating the environments.
A known method of underground gasification of a series of gently dipping seams (cf. U.S. Pat. No. 4,089,374 Cl. 166-259 published May 16, 1978) involves simultaneous gasification of two adjacent coal seams, which may cause interconnection of degasified seams due to a seam-spacing downfall and violation or termination of a gasification process. Another disadvantage of the aforesaid method is that, in simultaneous gasification of two or more seams, the surface of the earth may collapse and gas may penetrate to said surface, thereby contaminating the environments.
Another known method of gasifying a series of coal seams (cf. M. A. Iophis and I. A. Turchaninov, VNIIpodzemgas proceedings, No. 6, Moscow, 1962, pp 43-54, in Russian) comprises the steps of determining the depth of occurrence and the thickness of each coal seam in the series, opening each seam in the series by drilling inclined horizontal or inclined wells, interconnecting said wells within the coal seam, setting afire the coal, thus establishing gasification passages and gasifying each subsequent seam in the series on completion of gasification of the preceding seam by supplying a gaseous medium into the first wells and removing the produced gas from the second wells.
A disadvantage of the known method is that it may neither preclude penetration of a gaseous medium and gas to the surface of the earth in gasification of seams in a series nor prevent the associated contamination of the environments and downfalls.